1. Technical Field
The present invention relates to a technical field of signal combining in a mobile communications system.
2. Background Art
In the mobile communications system, a mobile terminal carried by a user and a base station apparatus are connected with each other by a communication line using a wireless communication medium (i.e. radio wave). The user of the mobile terminal can communicate with others while moving. Since the feature is attractive to the user, the mobile communications system is becoming widely used.
Generally, an important issue in the mobile communications system is to realize a communication system which uses energy (i.e. electric power) as little as possible when transmitting a signal via a wireless communication medium. In other words, it is a very important issue to reduce a required electric power when establishing a communication line via a wireless communication medium i.e. wireless communication link. That reason is that interference between a wireless communication link used by the user and a wireless communication link used by another user can be reduced by establishing the wireless communication link that uses a minimum electric power. As a result, an increase of a communication capacity can be advantageously obtained in a limited communication band.
As one of methods for reducing the required electric power in the wireless communication link, there is the method of improving the reception efficiency in the base station apparatus. An improvement of a reception efficiency means that an electric power of a transmission side (i.e. mobile terminal) that is required for obtaining a necessary signal-to-noise ratio (S/N ratio) can be reduced.
A so-called third generation mobile telecommunications system employs a code division multiple access (CDMA) communication method. A CDMA communication system employs a communication form that is different from a conventional access method (for example, frequency division multiplex access (FDMA) communication system, time division multiplex access (TDMA) communication system, or the like).
That is, in the CDMA communication system, a plurality of wireless communication links are simultaneously established between the mobile terminal and the base station apparatus. In such communication, generally, an operation called as “soft handover” or “soft handoff” is performed.
A communication link between the mobile terminal and the base station apparatus includes “a down link” and “an up link”. The down link is a communication link for transmitting a wireless signal to the mobile terminal from the base station apparatus. On the other hand, the up link is a link for transmitting the wireless signal to the base station apparatus from the mobile terminal.
In a connection mode of the down link in the CDMA communication system, each of a plurality of base station apparatuses transmits a radio wave. One mobile terminal receives radio waves transmitted from the plurality of base station apparatuses. Since the above mentioned connection mode is used, a so-called RAKE reception method is employed in the CDMA communication system.
The mobile terminal employing the RAKE reception method receives signals from a plurality of base station apparatuses and one or more circuits therein separate received signals individually. The mobile terminal performs an adjustment so that the signals have the same arrival time, and also weight received signals so as to obtain a maximum reception gain. After that, the mobile terminal despreads the weighted signal to obtain a decoded baseband signal.
On the other hand, in a connection mode of the up link in the CDMA communication system, a radio wave transmitted by one mobile terminal is received by a plurality of base station apparatuses which exist around the mobile terminal. In the CDMA communication system, there are several methods for processing a wireless signal received by the base station apparatus with respect to an internal configuration of each base station apparatus.
In a first method, a plurality of base station apparatuses that are located at geographically dispersed locations individually receive a signal transmitted from the mobile terminal and also despread a received signal to obtain the decoded baseband signal. The signal that is individually decoded in each base station apparatus in the method is selected based on signal quality thereof. The method is called as “soft handoff”. After quality of each of a plurality of received signals in the process mentioned above is individually evaluated, only a normal signal is selected and the selected signals are combined to obtain a baseband signal. Such processes are called as “selection combining (SC)”.
A second method is a method using a so-called “sector base station”. A sector base station includes a plurality of antenna elements that is called as a sector antenna. A plurality of sector antennas of the sector base station individually receive a wireless signal transmitted from the mobile terminal and the sector base station adjusts the signal so that each arrival time of the signal received by the sector antenna becomes equal to each other. After that, the sector base station combines received signals whose arrival times are adjusted to be equal to each other. The second method is, in principle, the approximately same as RAKE method in a connection mode of the down link mentioned above (that is, a method in which downlink signals received from a plurality of base station apparatuses are combined in one mobile terminal). The method is called as “softer handoff”. Each of received signals whose arrival times are adjusted to be equal to each other is weighted so as to maximize reception gain. Such processes is called “maximum ratio combining (MRC)”. Each of received signals which is weighted in the sector base station is decoded to obtain the baseband signal.
Japanese Patent Application Laid-Open No. 2000-13289 discloses a method in which a soft handoff (selection combining) and a softer handoff (maximum ratio combining) are combined. In the method, signals received by a plurality of sector antennas (sector base stations) which employ a RAKE reception method is decoded to obtain the baseband signal. The decoded signal is compared with a signal decoded from the baseband signal based on a received signal in another base station apparatus. In the method, one of the decoded signals that are compared with each other is selected based on quality of the decoded signals. That is, a reception form of the RAKE reception method in the method is a reception form in which the softer handoff and the soft handoff in which a signal obtained by the softer handoff or a signal obtained by another base station apparatus is chosen according to quality information are combined.
A technology of which a wireless signal transmitted from one mobile terminal is received by a plurality of base station apparatuses or a plurality of sector antennas in the up link is called a diversity technology. By employing the diversity technology, even if a reception power level of a signal received by the base station apparatus of a plurality of base station apparatuses is temporarily reduced due to buildings or the like, a signal transmitted from the mobile terminal can be efficiently received by a signal received by another base station apparatuses. An effect in such diversity technology is called a space diversity effect.
However, with respect to a reception gain (reception efficiency) of the base station apparatus, an effect of the selection combining (SC) differs from that of the maximum ratio combining (MRC). Hereinafter, the two cases will be compared.
First, a first reception form is a case in which two base station apparatuses that are located at geographically dispersed locations receive a wireless signal, respectively, and perform selection combining processing based on the received wireless signals. A second reception form is a case in which two sector antennas provided in one sector base station receive a wireless signal, respectively and maximum ratio combining processing is performed based on the received wireless signals.
When selection combining (SC) processing is performed, it is necessary that a received power in at least one base station apparatus of a plurality of base station apparatuses meets a predetermined reception level that is required for a reception.
On the other hand, when maximum ratio combining processing (MRC) is performed, signal combining is performed by adding a plurality of received signals received by each sector antenna in one sector base station. For this reason, the maximum ratio combining (MRC) has feature, there is no necessity that a level of a signal before a combining that is received by each sector antenna meets the above predetermined reception level for the selection combining (SC).
In other words, the above feature means that in the up link, a reception gain of the maximum ratio combining method is generally higher than that of the selection combining method. That is, in a maximum ratio combining method, a good quality communication can be realized even if a transmission power of the mobile terminal for satisfying a required signal level is low in comparison with a required transmission power for the selection combining.
Accordingly, when the maximum ratio combining method is extensively used, a transmission power of the mobile terminal can be reduced. As a result, a capacity of the up link can be increased.
The maximum ratio combining method has an advantage of a high reception gain. However, when the maximum ratio combining processing (MRC) is performed, a communication line in the base station side which is used in order to perform signal combining has to be a high-speed line which is capable of transferring a signal without reducing a transmission rate of a signal in a wireless section. Since there are such restrictions, the maximum ratio combining method cannot be used for all the connection patterns between the mobile terminal and the base station apparatus (or a sector antenna). Here, the above restrictions will be described more specifically.
In a selection combining method, a signal with a several MHz bandwidth in a high frequency band in a wireless section is decoded, and also the baseband signal with about ten and several kHz bandwidth obtained by a decoding is produced. In the selection combining method, it is enough to transfer the baseband signal and a signal in which some amount of quality information is added to a point where a judgment of selection combining is performed. That is, in the selection combining method, a communication line with about ten and several kHz bandwidth is enough to transfer a signal to the point where a judgment of selection combining is performed.
In contrast, in the maximum ratio combining method, first, each sector antenna receives a signal with a several MHz bandwidth in the high frequency band in the wireless section. In the maximum ratio combining method, it is necessary to transfer the signal in the high frequency band to a point where a maximum ratio combining process is performed, without changing a transmission rate or with keeping the signal to be in an equal bandwidth. This is because, it is necessary to adjust an arrival time of each received signal and weight a reception gain as mentioned above in the maximum ratio combining method. For this reason, in the maximum ratio combining method, a high speed line whose speed is almost the same as that of the above wireless section is required for a communication line of a base station side.
Accordingly, for the reason mentioned above, generally, the maximum ratio combining method is employed in one base station (sector base station) which is capable of handling signals individually received by a plurality of antenna elements (sector antennas) with keeping a speed of the signal almost the same as a high speed signal in the wireless section. That is, the maximum ratio combining method is not employed for a plurality of base station apparatuses that are located at geographically dispersed locations.
However, in an actual situation, there are few cases in which a mobile terminal is located only in an area which is controlled by one base station and is overlapped with neighboring sectors (the area is a coverage area of the base station). In fact, a mobile terminal is frequently located in a coverage area of a plurality of base station apparatuses which face each other across the mobile terminal. That is, it is considered that it is often the case in which a mobile terminal is located in a plurality of base station areas that are located at geographically dispersed locations. Accordingly, in case that a mobile terminal exists in a plurality of base station areas, if a reception gain can be improved, as a result, a capacity of an up link can be increased.
Accordingly, it is technically possible to connect between a plurality of base station apparatuses that are located in geographically dispersed points with a high speed line whose transmission speed is almost the same as that of the wireless section, in order to increase a capacity of an up link. However, it is impractical to connect between all the neighboring base station apparatuses with the high speed line in a mesh when cost is taken into consideration.
However, a system configuration in which in an up link which transfers a signal from a mobile terminal to a base station apparatus, the maximum ratio combining (MRC) method is employed only between sector antennas that are adjacently arranged in one base station apparatus (sector base station) and meanwhile, the selection combining (SC) method is employed between different base station apparatuses that are located at other locations should be improved from a view point of improving an efficiency of a reception gain of an up link. Here, a present status and an issue of combining processing that is employed in such system configuration will be described more in detail with reference to FIG. 45, FIGS. 46A to 46C and FIGS. 47A to 47C.
FIG. 45 is a figure illustrating a system configuration in which various kinds of base station apparatuses are connected with each other in a related mobile communications system and a processing procedure of a received signal.
In FIG. 45, a base station apparatus 12020 is a sector base station having three sector antennas 12011, 12012 and 12013. The base station apparatus 12020 forms a base station area 121 as a coverage area by the three sector antennas.
On the other hand, a base station apparatus 12021 is a base station apparatus having a non-directional antenna (for example, omni-directional antenna etc.) and forms a base station area 122 as a coverage area.
When two base station apparatuses 12020 and 12021 are located at geographically dispersed locations as shown in FIG. 45, a mobile terminal 101 is located at a point on a boundary between the base station area 121 and the base station area 122. The mobile terminal 101 exists in an overlapped coverage area of two sector antennas 12012 and 12013 in the base station area 121, and also exists in a coverage area of the base station apparatus 12021 (base station area 122). Received signals from the sector antennas 12012 and 12013 are processed inside of the one base station apparatus (sector base station) 12020. Since a high speed line can be used for the received signals from the sector antennas inside the base station apparatus, a signal combining by the maximum ratio combining method can be applied.
More specifically, a maximum ratio combiner (CMB) 1301 is installed in the base station apparatus 12020 whose coverage area is the base station area 121. The maximum ratio combiner 1301 performs maximum ratio combining processing based on received signals from the three sector antennas (in a case shown in FIG. 45, received signals from the two sector antennas 12012 and 12013). The maximum ratio combiner 1301 decodes a baseband signal based on an output of maximum ratio combining processing and also adds predetermined quality information to the decoded signal. An output signal of the maximum ratio combiner (CMB) 1301 is transferred to a diversity handover trunk (DHT) 1302 in order to perform selection combining with a signal from an antenna provided in another base station apparatus.
On the other hand, in order to perform selection combining with a signal from an antenna provided in another base station apparatus, the base station apparatus 12021 decodes a signal received in the base station area 122 into a baseband signal, and also adds predetermined quality information to the decoded signal. An output signal of the base station apparatus 12021 is transferred to the diversity handover trunk (DHT) 1302.
The diversity handover trunk 1302 receives a baseband signal decoded in the base station apparatus covering the base station area 121 and receives a baseband signal decoded in the base station apparatus covering the base station area 122. Moreover, by referring to the quality information that attaches to each received baseband signal, the diversity handover trunk 1302 selects one of two received baseband signals based on the quality information. The selected signal is sent to a RNC (radio network controller) 1307.
In case of an example shown in FIG. 45, with respect to quality of a signal transmitted from the mobile terminal 101, it is necessary that a quality of either one or both of two signals that are inputted to the diversity handover trunk 1302 has to satisfy a predetermined level. Here, the two signals include a signal obtained by a process in which signals received by the sector antennas 12012 and 12013 are processed by a maximum ratio combining and the processed signal is decoded into a baseband signal and a signal obtained by a process in which a signal received by the base station apparatus 12021 is decoded into a baseband signal.
However, a higher reception gain can be obtained by combining all the signals received by the sector antennas 12012 and 12013 and the base station apparatus 12021 by using the maximum ratio combining method. A reason why such method can not be actually applied is that a high speed line which can perform maximum ratio combining processing in an external apparatus for a plurality of base station apparatuses which are installed at geographically dispersed locations is not available.
That is, in an up link, in order to perform maximum ratio combining (MRC) processing based on outputs from receiving antennas as many as possible (output signal of a base station apparatus), it is desirable to prepare all the high speed lines in advance by considering all connection patterns that are configured between a mobile terminal and a base station apparatus or a sector antenna according to a location of a mobile terminal that exists in a service area.
In this case, a base station apparatus having a non-directional antenna or a base station having a sector antenna (hereinafter, it is abbreviated as “base station apparatus group”) to be connected by a high speed line forms a connection pattern of which a plurality of coverage areas formed by the base station apparatus group are neighboring. As another connection pattern, in the base station apparatus group which should be connected by a high speed line, one or more base station apparatus groups exist between neighboring coverage areas and as a result, a connection pattern is a pattern of which all the coverage areas are close.
However, when a connection pattern of the base station apparatus group is selected according to the above judgment criteria, even when it is visually judged that both coverage areas are neighboring or close, actually, there is a case that it is not effective to perform maximum ratio combining processing caused by an influence of a position or a height of a structure which can be an obstacle to a radio wave.
Accordingly, it is not cost-effective to prepare an expensive high speed line in advance by considering all the connection patterns. Moreover, if these high speed lines are permanently established, communication lines are wastefully left almost unused. Accordingly, it is required to establish only a high speed line truly required.
Generally, an area where a mobile communications system is largely used is a congested urban area or a densely populated area. However, in the areas, a distribution condition of users of a mobile terminal changes at every moment. For example, since users move for their reasons according to some conditions such as in a weekday morning, in a daytime, in an evening, late at night, a weekend or a special event, a distribution condition of a plurality of mobile terminals changes according to the movement of users. For this reason, it is necessary to reduce an interference power of an up link by realizing a reception using the maximum ratio combining method among base station apparatuses having the non-directional antenna and base stations having the sector antenna as many as possible according to a movement or distribution conditions of users (mobile terminals).
Here, in the mobile communications system shown in FIG. 45, a transition of a connection state of a communication line when a mobile terminal moves will be described with reference to FIGS. 46A to 46C and FIGS. 47A to 47C.
FIGS. 46A to 46C are figures generally illustrating a connection mode of a maximum ratio combiner and a selection combiner in a mobile communications system shown in FIG. 45. Further, in FIGS. 46A to 46C, the maximum ratio combiner (CMB) 1301 shown in FIG. 45 is represented as a maximum ratio combining point (MRC-P) 1305. The diversity handover trunk (DHT) 1302 shown in FIG. 45 is represented as a selection combining point (SC-P) 1306.
FIG. 46A shows a situation in which the mobile terminal 101 approaches sector antennas 12012 and 12013 and also approaches the base station apparatus 12021. In such a case, the maximum ratio combining point 1305 provided in the sector base station 12020 performs maximum ratio combining processing to signals received by the sector antennas 12012 and 12013. The selection combining point 1306 performs selection combining processing based on a signal combined by the maximum ratio combining point 1305 and a signal received by the base station apparatus (base station apparatus having a non-directional antenna) 12021.
Next, FIG. 46B shows a situation in which the mobile terminal 101 moves to another position that is different from the position shown in FIG. 46A. That is, in FIG. 46B, the mobile terminal 101 slightly moves away from the coverage areas by base station apparatus 12021 compared with the position shown in FIG. 46A and it is located on a boundary between coverage areas by sector antennas 12012 and 12013. In this case, it is effective to apply maximum ratio combining processing to the signals received by the sector antennas 12012 and 12013 in the maximum ratio combining point 1305. However, in the situation, it is not effective to perform selection combining processing using an output signal of the maximum ratio combining point 1305 and an output signal of the base station apparatus 12021. Thus, in the situation shown in FIG. 46B, an output signal of the base station apparatus 12021 is excluded from an input source for a signal combining.
FIG. 46C shows a situation in which the mobile terminal 101 moves to another position that is different from the position shown in FIG. 46A and FIG. 46B mentioned above. FIG. 46C shows the situation in which the mobile terminal 101 is located on a boundary of the base station area 122 of the base station apparatus 12021 and on a boundary of the sector antenna 12013 (base station area 120, further, it is located on a position which is away from the sector antenna 12012. Then, the selection combining point 1306 performs selection combining processing based on an output signal of the sector antenna 12013 and a signal received by the base station apparatus 12021.
Hereinafter, a difference in signal processing for respective situations that are shown in FIGS. 46A to 46C mentioned above will be described with reference to FIGS. 47A to 47C.
FIGS. 47A to 47C are figures logically illustrating a relation of a communication connection of base station apparatuses 12020 and 12021, the maximum ratio combining point 1305 and the selection combining point 1306 in the situation shown in FIGS. 46A to 46C.
FIG. 47A is a figure logically showing a connection situation shown in FIG. 46A mentioned above. In FIG. 47A, a bold line connecting the sector antennas 12012, 12013 and the maximum ratio combining point 1305 represents a high speed line which can transfer a signal for performing maximum ratio combining processing. That is, a high speed line is used for a communication line which connects each sector antenna and the maximum ratio combining point 1305 in the sector base station 12020.
Here, the high speed line (i.e. the bold line in FIG. 47A) provided in the sector base station 12020 is a line which has almost the same performance as a transmission link which is capable of transmitting a signal with a several MHz bandwidth in a high frequency band in a wireless section as mentioned in a description of the maximum ratio combining method.
The maximum ratio combining point 1305 performs maximum ratio combining processing based on a signal received by the sector antennas 12012 and 12013 and also transfers a signal that represents a result of the processing to the selection combining point 1306. The selection combining point 1306 performs selection combining processing based on a signal received from the maximum ratio combining point 1305 and a signal received from the base station apparatus 12021. Since it is a precondition that a low speed line is used for each communication line which transfers a signal used for selection combining, the communication line is indicated by a thin line.
Here, a low speed line indicated by a thin line is a line whose transmission speed is very low compared with that of a high speed line with a several MHz bandwidth as mentioned above in a description of the selection combining method.
FIG. 47B is a figure logically illustrating a connection situation shown in FIG. 46B mentioned above. In this case, since maximum ratio combining processing is performed by using a signal received from the sector antennas 12012, 12013, a line connecting the sector antenna and the maximum ratio combining point 1305 is expressed by the same bold line as FIG. 47A. However, a signal received by the base station apparatus 12021 is excluded from an input source due to a positional relation of the mobile terminal 101 shown in FIG. 46. Thus, a communication line (low speed line) between the maximum ratio combining point 1305 and the selection combining point 1306 is not established. That is, the selection combining point 1306 does not perform selection combining processing.
FIG. 47C is a figure logically illustrating a connection situation shown in FIG. 46C mentioned above. Only a signal received by the sector antenna 12013 is used in the sector base station 12020 due to a positional relation of the mobile terminal 101 shown in FIG. 46C. Then, a low speed line which connects the sector antenna 12013 and the selection combining point 1306 and the low speed line which connects the base station apparatus 12021 and the selection combining point 1306 are established. The selection combining point 1306 performs selection combining processing based on signals received via the low speed lines.
In FIGS. 47A to 47C, a high speed line (bold line) which transfers a signal used for maximum ratio combining processing and the low speed line (thin line) which transfers a signal used for selection combining are selectively established to each sector antenna appropriately according to a positional relation of the mobile terminal 101. On the other hand, with respect to the base station apparatus 12021 having one non-directional antenna, only the low speed line (thin line) which transfers a signal used for selection combining is established.
As an example mentioned above, it is necessary to transfer a signal received in each antenna element to a maximum ratio combining point (maximum ratio combiner) via the high speed line whose transmission speed is almost the same as that of a wireless section to perform maximum ratio combining processing. For this reason, in a general system configuration, maximum ratio combining processing is performed only in a sector base station in which the high speed line is available. In this case, maximum ratio combining processing is performed only between a plurality of sector antennas accommodated in one sector base station. That is, in a conventional mobile communications system, maximum ratio combining processing cannot be performed between a plurality of base station apparatuses which are located at geographically dispersed locations.